Coordination Complexes Containing Monosulfonated Catecholate Ligands and Methods for Producing the Same

ABSTRACT

Flow batteries and other electrochemical systems can contain an active material that is a coordination complex having at least one monosulfonated catecholate ligand or a salt thereof bound to a metal center. The monosulfonated catecholate ligand has a structure of 
     
       
         
         
             
             
         
       
     
     More particularly, the coordination complex can be a titanium coordination complex with a formula of D g Ti(L 1 )(L 2 )(L 3 ), in which D is a counterion selected from H, NH 4   + , Li + , Na + , K + , or any combination thereof; g ranges between 3 and 6; and L 1 , L 2  and L 3  are ligands, where at least one of L 1 , L 2  and L 3  is a monosulfonated catecholate ligand. Methods for synthesizing such monosulfonated catecholate ligands can include providing a neat mixture of catechol and up to about 1.3 stoichiometric equivalents of sulfuric acid, and heating the neat mixture at a temperature of about 80° C. or above to form 3,4-dihydroxybenzenesulfonic acid or a salt thereof.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.15/060,493, filed Mar. 6, 2016, the disclosure of which is incorporatedby reference herein.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

FIELD

The present disclosure generally relates to coordination complexes and,more specifically, to flow batteries and other electrochemical systemscontaining soluble coordination complexes as an active material.

BACKGROUND

Electrochemical energy storage systems, such as batteries,supercapacitors and the like, have been widely proposed for large-scaleenergy storage applications. Various battery designs, including flowbatteries, have been considered for this purpose. Compared to othertypes of electrochemical energy storage systems, flow batteries can beadvantageous, particularly for large-scale applications, due to theirability to decouple the parameters of power density and energy densityfrom one another.

Flow batteries generally include negative and positive active materialsin corresponding electrolyte solutions, which are flowed separatelyacross opposing sides of a membrane or separator in an electrochemicalcell containing negative and positive electrodes. The flow battery ischarged or discharged through electrochemical reactions of the activematerials that occur inside the two half-cells. As used herein, theterms “active material,” “electroactive material,” “redox-activematerial” or variants thereof will synonymously refer to materials thatundergo a change in oxidation state during operation of a flow batteryor like electrochemical energy storage system (i.e., during charging ordischarging). Although flow batteries hold significant promise forlarge-scale energy storage applications, they have often been plagued bysub-optimal energy storage performance (e.g., round trip energyefficiency) and limited cycle life, among other factors. Despitesignificant investigational efforts, no commercially viable flow batterytechnologies have yet been developed.

Metal-based active materials can often be desirable for use in flowbatteries and other electrochemical energy storage systems. Althoughnon-ligated metal ions (e.g., dissolved salts of a redox-active metal)can be used as an active material, it can often be more desirable toutilize coordination complexes for this purpose. As used herein, theterms “coordination complex, “coordination compound,” and “metal-ligandcomplex” will synonymously refer to a compound having at least onecovalent bond formed between a metal center and a donor ligand. Themetal center can cycle between an oxidized form and a reduced form in anelectrolyte solution, where the oxidized and reduced forms of the metalcenter represent states of full charge or full discharge depending uponthe particular half-cell in which the coordination complex is present.

A difficulty with coordination complexes, particularly those containingorganic ligands, is that they often can have relatively poor solubilitycharacteristics as a result of ligand hydrophobicity, particularly inaqueous media. Other factors such as packing and van der Waalsinteraction can also impact solubility characteristics. Poor solubilitycan result in sub-optimal performance of a flow battery due to the needto maintain a low concentration of active material in an electrolytesolution. Moreover, poor solubility of an active material can result inpotentially damaging precipitation within the various components of aflow battery system. For example, precipitation can occlude various flowpathways, foul membranes, and/or damage pumps within a flow batterysystem. Maintaining an electrolyte solution near an active material'ssaturation concentration to achieve good electrochemical performance canbe especially precarious due to these types of precipitation concerns.

Many electrolyte solutions containing coordination complexes can alsohave sub-optimal conductivity performance. Oftentimes, coordinationcomplexes themselves are non-ionic or only carry a minimal amount ofconductivity-promoting counterions. Moreover, because of the limitedsolubility of some coordination complexes, it can be difficult to add asufficient amount of an extraneous electrolyte (e.g., a non-redox activematerial) to an electrolyte solution to enhance conductivity to adesired degree. Specifically, adding an extraneous electrolyte to anelectrolyte solution can decrease the active material's saturationsolubility (e.g., through a common-ion affect), thereby decreasing theamount of charge that can be stored in a given volume of the electrolytesolution.

In view of the foregoing, active materials based upon high-solubilitycoordination complexes and methods for producing such complexes would behighly desirable in the art. The present disclosure satisfies theforegoing needs and provides related advantages as well.

SUMMARY

In some embodiments, the present disclosure provides compositionscontaining a coordination complex having at least one monosulfonatedcatecholate ligand or a salt thereof bound to a metal center. The atleast one monosulfonated catecholate ligand has a structure of

In other various embodiments, the present disclosure provides flowbatteries containing a first half-cell having a first electrolytesolution therein, where the first electrolyte solution contains anaqueous solution containing a coordination complex having at least onemonosulfonated catecholate ligand bound to a metal center. Morespecifically, the coordination complex can have a formula of

D_(g)Ti(L₁)(L₂)(L₃),

where D is a counterion selected from H, NH₄ ⁺, Li⁺, Na⁺, K⁺, or anycombination thereof; g ranges between 3 and 6; and L₁, L₂ and L₃ areligands. At least one of L₁, L₂ and L₃ is a monosulfonated catecholateligand.

In still other various embodiments, the present disclosure providesmethods for synthesizing monosulfonated catecholate ligands. The methodsinclude providing a neat mixture of catechol and up to about 1.3stoichiometric equivalents of sulfuric acid, and heating the neatmixture at a temperature of about 80° C. or above to form a reactionproduct containing 3,4-dihydroxybenzenesulfonic acid or a salt thereof.

The foregoing has outlined rather broadly the features of the presentdisclosure in order that the detailed description that follows can bebetter understood. Additional features and advantages of the disclosurewill be described hereinafter. These and other advantages and featureswill become more apparent from the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

For a more complete understanding of the present disclosure, and theadvantages thereof, reference is now made to the following descriptionsto be taken in conjunction with the accompanying drawings describingspecific embodiments of the disclosure, wherein:

FIG. 1 shows a schematic of an illustrative flow battery;

FIG. 2 shows an illustrative ¹H NMR spectrum in D₂O of the aromaticregion of 3,4-dihydroxybenzenesulfonic acid following purification; and

FIG. 3 shows an illustrative ¹H NMR spectrum in D₂O of the aromaticregion of the titanium coordination complex formed from 2 equivalents ofcatechol and 1 equivalent of 3,4-dihydroxybenzenesulfonic acid.

DETAILED DESCRIPTION

The present disclosure is directed, in part, to flow batteries andcompositions containing coordination complexes having at least onemonosulfonated catecholate ligand bound to a metal center. The presentdisclosure is also directed, in part, to methods for synthesizingmonosulfonated catecholate ligands, specifically3,4-dihydroxybenzenesulfonic acid (4-catecholsulfonic acid) or a saltthereof, and coordination complexes containing these ligands.

The present disclosure may be understood more readily by reference tothe following description taken in connection with the accompanyingfigures and examples, all of which form a part of this disclosure. It isto be understood that this disclosure is not limited to the specificproducts, methods, conditions or parameters described and/or shownherein. Further, the terminology used herein is for purposes ofdescribing particular embodiments by way of example only and is notintended to be limiting unless otherwise specified. Similarly, unlessspecifically stated otherwise, any description herein directed to acomposition is intended to refer to both solid and liquid versions ofthe composition, including solutions and electrolytes containing thecomposition, and electrochemical cells, flow batteries, and other energystorage systems containing such solutions and electrolytes. Further, itis to be recognized that where the disclosure herein describes anelectrochemical cell, flow battery, or other energy storage system, itis to be appreciated that methods for operating the electrochemicalcell, flow battery, or other energy storage system are also implicitlydescribed.

It is also to be appreciated that certain features of the presentdisclosure may be described herein in the context of separateembodiments for clarity purposes, but may also be provided incombination with one another in a single embodiment. That is, unlessobviously incompatible or specifically excluded, each individualembodiment is deemed to be combinable with any other embodiment(s) andthe combination is considered to represent another distinct embodiment.Conversely, various features of the present disclosure that aredescribed in the context of a single embodiment for brevity's sake mayalso be provided separately or in any sub-combination. Finally, while aparticular embodiment may be described as part of a series of steps orpart of a more general structure, each step or sub-structure may also beconsidered an independent embodiment in itself.

Unless stated otherwise, it is to be understood that each individualelement in a list and every combination of individual elements in thatlist is to be interpreted as a distinct embodiment. For example, a listof embodiments presented as “A, B, or C” is to be interpreted asincluding the embodiments “A,” “B,” “C,” “A or B,” “A or C,” “B or C,”or “A, B, or C.”

In the present disclosure, the singular forms of the articles “a,” “an,”and “the” also include the corresponding plural references, andreference to a particular numerical value includes at least thatparticular value, unless the context clearly indicates otherwise. Thus,for example, reference to “a material” is a reference to at least one ofsuch materials and equivalents thereof.

In general, use of the term “about” indicates approximations that canvary depending on the desired properties sought to be obtained by thedisclosed subject matter and is to be interpreted in a context-dependentmanner based on functionality. Accordingly, one having ordinary skill inthe art will be able to interpret a degree of variance on a case-by-casebasis. In some instances, the number of significant figures used whenexpressing a particular value may be a representative technique ofdetermining the variance permitted by the term “about.” In other cases,the gradations in a series of values may be used to determine the rangeof variance permitted by the term “about.” Further, all ranges in thepresent disclosure are inclusive and combinable, and references tovalues stated in ranges include every value within that range.

As discussed above, energy storage systems that are operable on a largescale while maintaining high efficiency values can be extremelydesirable. Flow batteries have generated significant interest in thisregard, but there remains considerable room for improving theiroperating characteristics. Although coordination complexes have beenexplored for use as active materials within flow batteries, the limitedsolubility of coordination complexes can sometimes be problematic andalso result in electrolyte solutions having low conductivity values.High aqueous solubility values can be particularly difficult to achieve.Exemplary description of illustrative flow batteries, their use, andoperating characteristics is provided hereinbelow.

Coordination complexes containing at least one catecholate ligand can beparticularly desirable active materials for use in flow batteries andother electrochemical systems. As used herein, the term “catechol” willrefer to a compound having an aromatic ring bearing hydroxyl groups onadjacent carbon atoms (i.e., 1,2-hydroxyl groups). Optional substitutioncan also be present in addition to the 1,2-hydroxyl groups. As usedherein, the term “catecholate” will refer to a substituted orunsubstituted catechol compound that is bound to a metal center via ametal-ligand bond. Like many other organic ligands, the relativelyhydrophobic nature of common catecholate ligands and the resultant lowsolubility of their coordination complexes can be problematic for thereasons discussed above. Other factors can also lead to problematicsolubility performance in some cases.

The present inventors recognized that the energy density and otheroperating parameters of flow batteries and related electrochemicalsystems could be improved by increasing the solubility of catecholatecoordination complexes while maintaining their desirable electrochemicalproperties. To this end, the inventors discovered that monosulfonatedcatecholate ligands can improve the solubility of coordination complexeswhile maintaining desirable electrochemical properties that are at leastcomparable to those of coordination complexes containing non-sulfonatedcatecholate ligands, including non-functionalized catecholate ligands.Titanium coordination complexes containing at least one monosulfonatedcatecholate ligand can be particularly desirable for this purpose. Asused herein, the term “monosulfonated catecholate ligand” will refer toa substituted catecholate ligand bearing one sulfonic acid group or anysalt thereof.

Although monosulfonated catecholate ligands can form coordinationcomplexes having increased solubility and desirable electrochemicalproperties, the inventors surprisingly found that further sulfonic acidsubstitution on the catecholate aromatic ring can be problematic. Forinstance, in the case of titanium, the inventors found that titaniumcatecholate complexes containing at least one disulfonated catecholateligand (e.g., 4,5-dihydroxy-1,3-benzenedisulfonic acid) were unstableunder the operating conditions of a flow battery. In contrast,corresponding titanium coordination complexes containing at least onemonosulfonated catecholate ligand remained stable under similarconditions.

Whereas 4,5-dihydroxy-1,3-benzenedisulfonic acid (trade name TIRON) is adisulfonated catecholate ligand that is commercially available,corresponding monosulfonated catecholate ligands having sulfonic acidsubstitution in either the 1- or the 3-position of the catechol aromaticring are not commercially available. In fact, there is only scantmention of such compounds in the chemical literature, and the knownprocesses for their synthesis are generally low-yielding, providedifficult-to-separate reaction mixtures, and/or, depending onconditions, result in non-regioselective reactivity at the 1- and the3-positions of the aromatic ring. Using presently available syntheticmethods, it can be particularly difficult to introduce a sulfonic acidfunctionality at the 3-position of the aromatic ring (i.e.,3,4-dihydroxybenzenesulfonic acid) without producing significant amountsof the other regioisomer (i.e., 2,3-dihydroxybenzenesulfonic acid) orproducing side products that can be undesirable for incorporation in anelectrolyte solution. As a result, presently available synthetic methodsfor producing monosulfonated catecholate ligands can be unsuitable foruse in conjunction with large-scale operations.

The present inventors also discovered a convenient and scalablesynthetic method for producing monosulfonated catecholate ligands with ahigh degree of regioselectivity. In particular, the inventors discoveredthat by reacting a neat mixture of catechol and a near-equivalent to asub-stoichiometric amount of sulfuric acid together with one another,predominantly monosulfonated catechol ligands could be produced. Byheating the neat mixture to a sufficiently high temperature,predominantly 3,4-dihydroxybenzenesulfonic acid can be formed. Incontrast, at or near room temperature, otherwise similar reactionconditions form a reaction product also containing a significantfraction of 2,3-dihydroxybenzenesulfonic acid.

If needed, the 3,4-dihydroxybenzenesulfonic acid can be isolated fromthe reaction product and undergo further complexation with a metalcenter to produce a coordination complex. In some instances, a mixtureof unreacted catechol and 3,4-dihydroxybenzenesulfonic acid can beisolated from the reaction product and undergo further complexation witha metal center to form a catecholate coordination complex having bothsulfonated catecholate ligands and non-sulfonated catecholate ligands.In either configuration, the monosulfonated catecholate ligands of thepresent disclosure can advantageously behave similarly to unsubstitutedcatecholate ligands in their reactivity toward titanium and other metalcenters. Hence, techniques conventionally used for synthesizing andpurifying catecholate coordination complexes can be applied similarlywhen utilizing monosulfonated catecholate ligands. Furtheradvantageously, coordination complexes containing monosulfonatedcatecholate ligands can also be readily obtained with mixed counterions(e.g., as a mixture of Na⁺ and K⁺ counterions), which can likewise bedesirable for enhancing their solubility.

In addition to improved solubility, coordination complexes containing atleast one sulfonated catecholate ligand can provide further advantagesas well. In particular, the highly ionized sulfonic acid group canimprove the ionic conductivity of electrolyte solutions in which suchcoordination complexes are present. By utilizing the coordinationcomplexes of the present disclosure, one can avoid adding an extraneouselectrolyte to electrolyte solutions in which the coordination complexesare present, or the amount of extraneous electrolyte can besignificantly decreased. Not only can omission or decrease in the amountof extraneous electrolyte reduce cost of goods, but it can alsoultimately allow higher concentrations of the active material to bepresent in the electrolyte solution, as discussed above. Decreasedcrossover of the charged active material across the separator of a flowbattery can also result.

In various embodiments, the present disclosure describes compositionsand flow batteries containing a coordination complex having at least onemonosulfonated catecholate ligand or a salt thereof bound to a metalcenter. In particular, the at least one monosulfonated catecholateligand can have a structure of

As indicated above, this monosulfonated catecholate ligand(3,4-dihydroxybenzenesulfonic acid or 3-catecholsulfonic acid) can bereadily obtained by the methods disclosed herein and discussed in moredetail below. Further disclosure regarding flow batteries containingsuch coordination complexes and their operating characteristics are alsodiscussed in more detail below.

In some embodiments, the metal center of the coordination complexesdisclosed herein can be a transition metal. Due to their variableoxidation states, transition metals can be highly desirable for usewithin the active material of a flow battery. Cycling between theaccessible oxidation states can result in the conversion of chemicalenergy into electrical energy. Lanthanide metals can be used similarlyin this regard in alternative embodiments. In general, any transitionmetal or lanthanide metal can be present as the metal center in thecoordination complexes of the present disclosure. In more specificembodiments, the metal center can be a transition metal selected fromamong Al, Cr, Ti and Fe. For purposes of the present disclosure, Al isto be considered a transition metal. In more specific embodiments, thetransition metal can be Ti. Other suitable transition and main groupmetals that can be present in the coordination complexes of the presentdisclosure include, for example, Ca, Ce, Co, Cu, Mg, Mn, Mo, Ni, Pd, Pt,Ru, Sr, Sn, V, Zn, Zr, and any combination thereof. In variousembodiments, the coordination complexes can include a transition metalin a non-zero oxidation state when the transition metal is in both itsoxidized and reduced forms. Cr, Fe, Mn, Ti and V can be particularlydesirable in this regard.

In more specific embodiments, coordination complexes of the presentdisclosure can have a formula of

D_(g)M(L₁)(L₂)(L₃),

where M is a transition metal; D is a counterion selected from H⁺, NH₄⁺, tetraalkylammonium (C₁-C₄ alkyl), an alkali metal ion (e.g., Li⁺, Na⁺or K⁺), or any combination thereof; g ranges between 1 and 8; and L₁, L₂and L₃ are ligands and at least one of L₁, L₂ and L₃ is a monosulfonatedcatecholate ligand as specified hereinabove.

In still more specific embodiments, coordination complexes of thepresent disclosure can have a formula of

D_(g)Ti(L₁)(L₂)(L₃),

where D is a counterion selected from H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, or anycombination thereof; g ranges between 3 and 6; and L₁, L₂ and L₃ areligands and at least one of L₁, L₂ and L₃ is a monosulfonatedcatecholate ligand as specified hereinabove. In some embodiments, D canbe chosen from among Li⁺, Na⁺, K⁺, or any combination thereof, and insome more specific embodiments, D can be a mixture of Na⁺ and K⁺counterions.

In some embodiments, the coordination complexes can have a formula suchthat each of L₁, L₂ and L₃ is a monosulfonated catecholate ligand, inwhich case g can be 5 or 6 to maintain charge balance.

In other embodiments, the coordination complexes can have a formula suchthat one of L₁, L₂ and L₃ is a monosulfonated catecholate ligand, inwhich case g can be 3 or 4 to maintain charge balance, provided that thenet ionic charge for the remainder of L₁, L₂ and L₃ is zero when theligands are bound to the metal center. In more specific embodiments, twoof L₁, L₂ and L₃ can be non-sulfonated catecholate ligands, and in stillmore specific embodiments, two of L₁, L₂ and L₃ can be unsubstitutedcatecholate ligands.

In still other embodiments, the coordination complexes can have aformula such two of L₁, L₂ and L₃ are monosulfonated catecholateligands, in which case g can be 4 or 5 to maintain charge balance,provided that the net ionic charge for the remainder of L₁, L₂ and L₃ iszero when the ligands are bound to the metal center. In more specificembodiments, one of L₁, L₂ and L₃ can be anon-sulfonated catecholateligand, and in still more specific embodiments, one of L₁, L₂ and L₃ canbe an unsubstituted catecholate ligand.

In some embodiments, the coordination complexes of the presentdisclosure can contain a mixture of counterions. As indicated above, amixture of counterions can desirably further improve the solubility ofthe coordination complexes. In more specific embodiments, thecoordination complexes of the present disclosure can have an overallnegative charge and contain a mixture of both Na⁺ and K⁺ counterions.Accordingly, in some embodiments, coordination complexes of the presentdisclosure can have a formula of

Na_(x)K_(y)Ti(L₁)(L₂)(L₃),

where 3≤x+y≤6, and at least one of L₁, L₂ and L₃ is a monosulfonatedcatecholate ligand as defined hereinabove. Both x and y are real numbersthat are greater than 0, and they can be equal or non-equal to oneanother. In some embodiments, substantially equimolar amounts of Na⁺ andK⁺ counterions can be present, such that x and y are substantially equalto one another. The values of both x and y are not necessarily integers,although they can be in some embodiments.

In some embodiments, other ligands can be present in the coordinationcomplexes in combination with monosulfonated catecholate ligands,optionally in further combination with non-sulfonated catecholateligands, including unsubstituted catecholate ligands. Other ligands thatcan be present in the coordination complexes include, for example,ascorbate, citrate, glycolate, a polyol, gluconate, hydroxyalkanoate,acetate, formate, benzoate, malate, maleate, phthalate, sarcosinate,salicylate, oxalate, urea, polyamine, aminophenolate, acetylacetonate,and lactate. Where chemically feasible, it is to be recognized that suchligands can be optionally substituted with at least one group selectedfrom among C₁₋₆ alkoxy, C₁₋₆ alkyl, C₁₋₆ alkenyl, C₁₋₆ alkynyl, 5- or6-membered aryl or heteroaryl groups, a boronic acid or a derivativethereof, a carboxylic acid or a derivative thereof, cyano, halide,hydroxyl, nitro, sulfonate, a sulfonic acid or a derivative thereof, aphosphonate, a phosphonic acid or a derivative thereof, or a glycol,such as polyethylene glycol. Alkanoate includes any of the alpha, beta,and gamma forms of these ligands. Polyamines include, but are notlimited to, ethylenediamine, ethylenediamine tetraacetic acid (EDTA),and diethylenetriamine pentaacetic acid (DTPA).

Other examples of ligands that can be present in the coordinationcomplexes in combination with at least one monosulfonated catecholateligand and/or any of the other aforementioned ligands can includemonodentate, bidentate, and/or tridentate ligands. Examples ofmonodentate ligands that can be present in the coordination complexes ofthe present disclosure include, for example, carbonyl or carbonmonoxide, nitride, oxo, hydroxo, water, sulfide, thiols, pyridine,pyrazine, and the like. Examples of bidentate ligands that can bepresent in the coordination complexes of the present disclosure include,for example, bipyridine, bipyrazine, ethylenediamine, diols (includingethylene glycol), and the like. Examples of tridentate ligands that canbe present in the coordination complexes of the present disclosureinclude, for example, terpyridine, diethylenetriamine, triazacydononane,tris(hydroxymethyl)aminomethane, and the like.

In further embodiments, the compositions of the present disclosure caninclude an aqueous solution in which the coordination complex isdissolved. That is, in some embodiments, aqueous solutions ofcoordination complexes containing at least one monosulfonatedcatecholate ligand are also expressly disclosed herein. Such aqueoussolutions can be employed as at least one of the electrolyte solutionsin a flow battery or a related electrochemical system. Furtherdisclosure regarding the aqueous solutions and their incorporation inflow batteries is provided hereinafter.

As used herein, the term “aqueous solution” will refer to a homogeneousliquid phase with water as a predominant solvent in which a coordinationcomplex of the present disclosure is at least partially solubilized,ideally fully solubilized. This definition encompasses both solutions inwater and solutions containing a water-miscible organic solvent as aminority component of an aqueous phase.

Illustrative water-miscible organic solvents that can be present in theaqueous solution include, for example, alcohols and glycols, optionallyin the presence of one or more surfactants or other components discussedbelow. In more specific embodiments, the aqueous solution can contain atleast about 98% water by weight. In other more specific embodiments, theaqueous solution can contain at least about 55% water by weight, or atleast about 60% water by weight, or at least about 65% water by weight,or at least about 70% water by weight, or at least about 75% water byweight, or at least about 80% water by weight, or at least about 85%water by weight, or at least about 90% water by weight, or at leastabout 95% water by weight. In some embodiments, the aqueous solution canbe free of water-miscible organic solvents and consist of water alone asa solvent.

In further embodiments, the aqueous solution can include a viscositymodifier, a wetting agent, or any combination thereof. Suitableviscosity modifiers can include, for example, corn starch, corn syrup,gelatin, glycerol, guar gum, pectin, and the like. Other suitableexamples will be familiar to one having ordinary skill in the art.Suitable wetting agents can include, for example, various non-ionicsurfactants and/or detergents. In some or other embodiments, the aqueoussolution can further include a glycol or a polyol. Suitable glycols caninclude, for example, ethylene glycol, diethylene glycol, andpolyethylene glycol. Suitable polyols can include, for example,glycerol, mannitol, sorbitol, pentaerythritol, andtris(hydroxymethyl)aminomethane. Inclusion of any of these components inthe aqueous solution can help promote dissolution of the coordinationcomplex and/or reduce viscosity of the aqueous solution for conveyancethrough a flow battery, for example.

In illustrative embodiments, the aqueous solution can have an alkalinepH. Alkaline pH values can be particularly desirable for promotingstability of coordination complexes containing catecholate ligands. Inaddition, alkaline pH values can maintain the sulfonic acid group ofsulfonated catecholate ligands in a deprotonated state, thereby furtherenhancing solubility. As used herein, the term “alkaline pH” will referto any pH value between about 7 and about 14. In some embodiments, oneor more buffers can be present in the aqueous solution to help maintainthe pH at an alkaline pH value. In more specific embodiments, theaqueous solution can be maintained at a pH of about 9 to about 12. A pHvalue residing within a range of about 9 to about 12 can be particularlydesirable for maintaining the phenolic groups of catecholate ligands ina deprotonated state and complexed to the metal center of thecoordination complex. Other illustrative alkaline pH ranges that can bemaintained in the aqueous solutions include, for example, about 7 toabout 7.5, or about 7.5 to about 8, or about 8 to about 8.5, or about8.5 to about 9, or about 9.5 to about 10, or about 10 to about 10.5, orabout 10.5 to about 11, or about 11 to about 11.5, or about 11.5 toabout 12, or about 12 to about 12.5, or about 12.5 to about 13, or about13 to about 13.5, or about 13.5 to about 14. Illustrative buffers thatcan be present include, but are not limited to, salts of phosphates,borates, carbonates, silicates, tris(hydroxymethyl)aminomethane (TRIS),4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), or any combinationthereof.

In addition to a solvent and a coordination complex as an activematerial, the aqueous solutions can also include one or more mobile ions(i.e., an extraneous electrolyte) for use as an electrolyte solution ina flow battery or similar electrochemical system. In some embodiments,suitable mobile ions can include proton, hydronium, or hydroxide. Inother various embodiments, mobile ions other than proton, hydronium, orhydroxide can be present, either alone or in combination with proton,hydronium or hydroxide. Such alternative mobile ions can include, forexample, alkali metal or alkaline earth metal cations (e.g., Li⁺, Na⁺,K⁺, Mg²⁺, Ca²⁺ and Sr²⁺) and halides (e.g., F⁻, Cl⁻, or Br⁻). Othersuitable mobile ions can include, for example, ammonium andtetraalkylammonium ions, chalcogenides, phosphate, hydrogen phosphate,phosphonate, nitrate, sulfate, nitrite, sulfite, perchlorate,tetrafluoroborate, hexafluorophosphate, and any combination thereof. Insome embodiments, less than about 50% of the mobile ions can constituteprotons, hydronium, or hydroxide. In other various embodiments, lessthan about 40%, less than about 30%, less than about 20%, less thanabout 10%, less than about 5%, or less than about 2% of the mobile ionscan constitute protons, hydronium, or hydroxide.

In other various embodiments, aqueous solutions containing acoordination complex having at least one monosulfonated catecholateligand can lack an extraneous electrolyte altogether. As indicatedabove, the highly ionized sulfonic acid group of the monosulfonatedcatecholate ligand(s) can provide sufficient ionic conductivity for useas a suitable electrolyte solution in many instances. In someembodiments, the aqueous solutions of the present disclosure can haveionic conductivity values up to about 80 mS/cm at 45° C. Theconductivity values can vary due to the concentration of the activematerial and/or due to the concentration of any extraneous electrolytesthat are present.

In various embodiments, a concentration of the coordination complex inthe aqueous solution can range between about 0.1 M and about 3 M. In anelectrolyte solution, this concentration range represents the sum of theindividual concentrations of the oxidized and reduced forms of thecoordination complex. In more particular embodiments, the concentrationof the coordination complex can range between about 0.5 M and about 3 M,or between 1 M and about 3 M, or between about 1.5 M and about 3 M, orbetween 1 M and about 2.5 M. Various solubility-promoting additives canlead to higher solubility values than are possible with the coordinationcomplex alone.

As indicated above, aqueous solutions of the present disclosure can beincorporated in flow batteries and related electrochemical systems.Further disclosure on suitable flow batteries and their operatingparameters follows hereinafter.

Accordingly, in various embodiments, flow batteries of the presentdisclosure can include a first half-cell having a first electrolytesolution therein, where the first electrolyte solution is an aqueoussolution containing a coordination complex having at least onemonosulfonated catecholate ligand or a salt thereof that is bound to ametal center, as defined hereinabove. In some embodiments, thecoordination complex can have a formula of

D_(g)M(L₁)(L₂)(L₃),

where M is a transition metal; D is a counterion selected from H⁺, NH₄⁺, tetraalkylammonium (C₁-C₄ alkyl), an alkali metal ion (e.g., Li⁺, Na⁺or K⁺), or any combination thereof; g ranges between 1 and 8; and L₁, L₂and L₃ are ligands and at least one of L₁, L₂ and L₃ is a monosulfonatedcatecholate ligand as specified hereinabove. In more specificembodiments, the coordination complex can be a titanium complex having aformula of

D_(g)Ti(L₁)(L₂)(L₃),

where D is a counterion selected from H⁺, NH₄ ⁺, Li⁺, Na⁺, K⁺, or anycombination thereof; g ranges between 3 and 6; and L₁, L₂ and L₃ areligands and at least one of L₁, L₂ and L₃ is a monosulfonatedcatecholate ligand as specified hereinabove. Additional disclosureregarding such coordination complexes is provided hereinabove.

In further embodiments, flow batteries of the present disclosure canalso include a second half-cell having a second electrolyte solutiontherein, where the second electrolyte solution contains an activematerial differing from that in the first electrolyte solution. In morespecific embodiments, the second electrolyte solution can be an aqueoussolution containing an iron hexacyanide complex. Iron hexacyanidecomplexes can be particularly desirable active materials due to theirfacile electrode kinetics and substantially reversible electrochemicalbehavior within the working electrochemical window of aqueous solutions.Hence, these complexes can allow high open circuit potentials and cellefficiencies to be realized, particularly in combination with titaniumcatecholate complexes as the active material in the first electrolytesolution. In more specific embodiments, flow batteries of the presentdisclosure can include the first electrolyte solution in contact with anegative electrode of the flow battery and the second electrolytesolution in contact with the positive electrode of the flow battery.

Illustrative flow battery configurations that can incorporate theforegoing electrolyte solutions and coordination complexes will now bedescribed in further detail. The flow batteries of the presentdisclosure are, in some embodiments, suited to sustained charge ordischarge cycles of several hour durations. As such, they can be used tosmooth energy supply/demand profiles and provide a mechanism forstabilizing intermittent power generation assets (e.g., from renewableenergy sources such as solar and wind energy). It should be appreciated,then, that various embodiments of the present disclosure include energystorage applications where such long charge or discharge durations aredesirable. For example, in non-limiting examples, the flow batteries ofthe present disclosure can be connected to an electrical grid to allowrenewables integration, peak load shifting, grid firming, baseload powergeneration and consumption, energy arbitrage, transmission anddistribution asset deferral, weak grid support, frequency regulation, orany combination thereof. When not connected to an electrical grid, theflow batteries of the present disclosure can be used as power sourcesfor remote camps, forward operating bases, off-grid telecommunications,remote sensors, the like, and any combination thereof. Further, whilethe disclosure herein is generally directed to flow batteries, it is tobe appreciated that other electrochemical energy storage media canincorporate the electrolyte solutions and coordination complexesdescribed herein, specifically those utilizing stationary electrolytesolutions.

In some embodiments, flow batteries of the present disclosure caninclude: a first chamber containing a negative electrode contacting afirst aqueous electrolyte solution; a second chamber containing apositive electrode contacting a second aqueous electrolyte solution, anda separator disposed between the first and second electrolyte solutions.The chambers provide separate reservoirs within the cell, through whichthe first and/or second electrolyte solutions circulate so as to contactthe respective electrodes and the separator. Each chamber and itsassociated electrode and electrolyte solution define a correspondinghalf-cell. The separator provides several functions which include, forexample, (1) serving as a barrier to mixing of the first and secondelectrolyte solutions, (2) electrically insulating to reduce or preventshort circuits between the positive and negative electrodes, and (3) tofacilitate ion transport between the positive and negative electrolytechambers, thereby balancing electron transport during charge anddischarge cycles. The negative and positive electrodes provide a surfacewhere electrochemical reactions can take place during charge anddischarge cycles. During a charge or discharge cycle, electrolytesolutions can be transported from separate storage tanks through thecorresponding chambers. In a charging cycle, electrical power can beapplied to the cell such that the active material contained in thesecond electrolyte solution undergoes a one or more electron oxidationand the active material in the first electrolyte solution undergoes aone or more electron reduction. Similarly, in a discharge cycle thesecond active material is reduced and the first active material isoxidized to generate electrical power.

In more specific embodiments, illustrative flow batteries of the presentdisclosure can include: (a) a first aqueous electrolyte solutioncontaining a first coordination complex; (b) a second aqueouselectrolyte solution containing a second coordination complex; (c) aseparator positioned between said first and second aqueous electrolytesolutions; and (d) an optional mobile ion in the first and secondaqueous electrolyte solutions. As described in more detail below, theseparator can be an ionomer membrane, and it can have a thickness ofless than 100 microns and have an associated net charge that is the samesign as that of the first and second coordination complexes.

FIG. 1 shows a schematic of an illustrative flow battery. Unlike typicalbattery technologies (e.g., Li-ion, Ni-metal hydride, lead-acid, and thelike), where active materials and other components are housed in asingle assembly, flow batteries transport (e.g., via pumping) redoxactive energy storage materials from storage tanks through anelectrochemical stack. This design feature decouples the electricalenergy storage system power from the energy storage capacity, therebyallowing for considerable design flexibility and cost optimization.

As shown in FIG. 1, flow battery system 1 includes an electrochemicalcell that features separator 20 (e.g., a membrane) that separates thetwo electrodes 10 and 10′ of the electrochemical cell. Electrodes 10 and10′ are formed from a suitably conductive material, such as a metal,carbon, graphite, and the like. Tank 50 contains first active material30, which is capable of being cycled between an oxidized state and areduced state.

Pump 60 affects transport of first active material 30 from tank 50 tothe electrochemical cell. The flow battery also suitably includes secondtank 50′ that contains second active material 40. Second active material40 can be the same material as active material 30, or it can bedifferent. Second pump 60′ can affect transport of second activematerial 40 to the electrochemical cell. Pumps can also be used toaffect transport of the active materials from the electrochemical cellback to tanks 50 and 50′ (not shown in FIG. 1). Other methods ofaffecting fluid transport, such as siphons, for example, can alsosuitably transport first and second active materials 30 and 40 into andout of the electrochemical cell. Also shown in FIG. 1 is power source orload 70, which completes the circuit of the electrochemical cell andallows a user to collect or store electricity during its operation.

It should be understood that FIG. 1 depicts a specific, non-limitingembodiment of a flow battery. Accordingly, flow batteries consistentwith the spirit of the present disclosure can differ in various aspectsrelative to the configuration of FIG. 1. As one example, a flow batterysystem can include one or more active materials that are solids, gases,and/or gases dissolved in liquids. Active materials can be stored in atank, in a vessel open to the atmosphere, or simply vented to theatmosphere.

As used herein, the terms “separator” and “membrane” refer to anionically conductive and electrically insulating material disposedbetween the positive and negative electrodes of an electrochemical cell.The separator can be a porous membrane in some embodiments and/or anionomer membrane in other various embodiments. In some embodiments, theseparator can be formed from an ionically conductive polymer.

Polymer membranes can be anion- or cation-conducting electrolytes. Wheredescribed as an “ionomer,” the term refers to polymer membranecontaining both electrically neutral repeating units and ionizedrepeating units, where the ionized repeating units are pendant andcovalently bonded to the polymer backbone. In general, the fraction ofionized units can range from about 1 mole percent to about 90 molepercent. For example, in some embodiments, the content of ionized unitsis less than about 15 mole percent and in other embodiments, the ioniccontent is higher, such as greater than about 80 mole percent. In stillother embodiments, the ionic content is defined by an intermediaterange, for example, in a range of about 15 to about 80 mole percent.Ionized repeating units in an ionomer can include anionic functionalgroups such as sulfonate, carboxylate, and the like. These functionalgroups can be charge balanced by, mono-, di-, or higher-valent cations,such as alkali or alkaline earth metals. Ionomers can also includepolymer compositions containing attached or embedded quaternaryammonium, sulfonium, phosphazenium, and guanidinium residues or salts.Suitable examples will be familiar to one having ordinary skill in theart.

In some embodiments, polymers useful as a separator can include highlyfluorinated or perfluorinated polymer backbones. Certain polymers usefulin the present disclosure can include copolymers of tetrafluoroethyleneand one or more fluorinated, acid-functional co-monomers, which arecommercially available as NAFION™ perfluorinated polymer electrolytesfrom DuPont. Other useful perfluorinated polymers can include copolymersof tetrafluoroethylene and FSO₂—CF₂CF₂CF₂CF₂—O—CF═CF₂, FLEMION™ andSELEMION™.

Additionally, substantially non-fluorinated membranes that are modifiedwith sulfonic acid groups (or cation exchanged sulfonate groups) canalso be used. Such membranes can include those with substantiallyaromatic backbones such as, for example, polystyrene, polyphenylene,biphenyl sulfone (BPSH), or thermoplastics such as polyetherketones andpolyethersulfones.

Battery-separator style porous membranes, can also be used as theseparator. Because they contain no inherent ionic conductioncapabilities, such membranes are typically impregnated with additives inorder to function. These membranes typically contain a mixture of apolymer and inorganic filler, and open porosity. Suitable polymers caninclude, for example, high density polyethylene, polypropylene,polyvinylidene difluoride (PVDF), or polytetrafluoroethylene (PTFE).Suitable inorganic fillers can include silicon carbide matrix material,titanium dioxide, silicon dioxide, zinc phosphide, and ceria.

Separators can also be formed from polyesters, polyetherketones,poly(vinyl chloride), vinyl polymers, and substituted vinyl polymers.These can be used alone or in combination with any previously describedpolymer.

Porous separators are non-conductive membranes which allow chargetransfer between two electrodes via open channels filled withelectrolyte. The permeability increases the probability of chemicals(e.g., active materials) passing through the separator from oneelectrode to another and causing cross-contamination and/or reduction incell energy efficiency. The degree of this cross-contamination candepend on, among other features, the size (the effective diameter andchannel length), and character (hydrophobicity/hydrophilicity) of thepores, the nature of the electrolyte, and the degree of wetting betweenthe pores and the electrolyte.

The pore size distribution of a porous separator is generally sufficientto substantially prevent the crossover of active materials between thetwo electrolyte solutions. Suitable porous membranes can have an averagepore size distribution of between about 0.001 nm and 20 micrometers,more typically between about 0.001 nm and 100 nm. The size distributionof the pores in the porous membrane can be substantial. In other words,a porous membrane can contain a first plurality of pores with a verysmall diameter (approximately less than 1 nm) and a second plurality ofpores with a very large diameter (approximately greater than 10micrometers). The larger pore sizes can lead to a higher amount ofactive material crossover.

The ability for a porous membrane to substantially prevent the crossoverof active materials can depend on the relative difference in sizebetween the average pore size and the active material. For example, whenthe active material is a metal center in a coordination complex, theaverage diameter of the coordination complex can be about 50% greaterthan the average pore size of the porous membrane. On the other hand, ifa porous membrane has substantially uniform pore sizes, the averagediameter of the coordination complex can be about 20% larger than theaverage pore size of the porous membrane. Likewise, the average diameterof a coordination complex is increased when it is further coordinatedwith at least one water molecule. The diameter of a coordination complexof at least one water molecule is generally considered to be thehydrodynamic diameter. In such embodiments, the hydrodynamic diameter isgenerally at least about 35% greater than the average pore size. Whenthe average pore size is substantially uniform, the hydrodynamic radiuscan be about 10% greater than the average pore size.

In some embodiments, the separator can also include reinforcementmaterials for greater stability. Suitable reinforcement materials caninclude nylon, cotton, polyesters, crystalline silica, crystallinetitania, amorphous silica, amorphous titania, rubber, asbestos, wood orany combination thereof

Separators within the flow batteries of the present disclosure can havea membrane thickness of less than about 500 micrometers, or less thanabout 300 micrometers, or less than about 250 micrometers, or less thanabout 200 micrometers, or less than about 100 micrometers, or less thanabout 75 micrometers, or less than about 50 micrometers, or less thanabout 30 micrometers, or less than about 25 micrometers, or less thanabout 20 micrometers, or less than about 15 micrometers, or less thanabout 10 micrometers. Suitable separators can include those in which theflow battery is capable of operating with a current efficiency ofgreater than about 85% with a current density of 100 mA/cm² when theseparator has a thickness of 100 micrometers. In further embodiments,the flow battery is capable of operating at a current efficiency ofgreater than 99.5% when the separator has a thickness of less than about50 micrometers, a current efficiency of greater than 99% when theseparator has a thickness of less than about 25 micrometers, and acurrent efficiency of greater than 98% when the separator has athickness of less than about 10 micrometers. Accordingly, suitableseparators include those in which the flow battery is capable ofoperating at a voltage efficiency of greater than 60% with a currentdensity of 100 mA/cm². In further embodiments, suitable separators caninclude those in which the flow battery is capable of operating at avoltage efficiency of greater than 70%, greater than 80% or even greaterthan 90%.

The diffusion rate of the first and second active materials through theseparator can be less than about 1×10⁻⁵ mol cm⁻² day⁻¹, or less thanabout 1×10⁻⁶ mol cm⁻² day⁻¹, or less than about 1×10⁻⁷ mol cm⁻² day⁻¹,or less than about 1×10⁻⁹ mol cm⁻² day⁻¹, or less than about 1×10⁻¹¹ molcm⁻² day⁻¹, or less than about 1×10⁻¹³ mol cm⁻² day⁻¹, or less thanabout 1×10⁻¹⁵ mol cm⁻² day⁻¹.

The flow batteries can also include an external electrical circuit inelectrical communication with the first and second electrodes. Thecircuit can charge and discharge the flow battery during operation.Reference to the sign of the net ionic charge of the first, second, orboth active materials relates to the sign of the net ionic charge inboth oxidized and reduced forms of the redox active materials under theconditions of the operating flow battery. The net ionic charge of thecoordination complexes disclosed herein can vary based upon the numberof deprotonated sulfonic acid groups that are present. Further exemplaryembodiments of a flow battery provide that (a) the first active materialhas an associated net positive or negative charge and is capable ofproviding an oxidized or reduced form over an electric potential in arange of the negative operating potential of the system, such that theresulting oxidized or reduced form of the first active material has thesame charge sign (positive or negative) as the first active material andthe ionomer membrane also has a net ionic charge of the same sign; and(b) the second active material has an associated net positive ornegative charge and is capable of providing an oxidized or reduced formover an electric potential in a range of the positive operatingpotential of the system, such that the resulting oxidized or reducedform of the second active material has the same charge sign (positive ornegative sign) as the second active material and the ionomer membranealso has a net ionic charge of the same sign; or both (a) and (b). Inthe case of the first active material being a coordination complexbearing one or more sulfonated catecholate ligands, the net ionic chargein both the oxidized and reduced forms can be negative. The matchingcharges of the first and/or second active materials and the ionomermembrane can provide a high selectivity. More specifically, chargematching can provide less than about 3%, less than about 2%, less thanabout 1%, less than about 0.5%, less than about 0.2%, or less than about0.1% of the molar flux of ions passing through the ionomer membrane asbeing attributable to the first or second active material. The term“molar flux of ions” will refer to the amount of ions passing throughthe ionomer membrane, balancing the charge associated with the flow ofexternal electricity/electrons. That is, the flow battery is capable ofoperating or operates with the substantial exclusion of the activematerials by the ionomer membrane, and such exclusion can be promotedthrough charge matching.

Flow batteries incorporating the electrolyte solutions of the presentdisclosure can have one or more of the following operatingcharacteristics: (a) where, during the operation of the flow battery,the first or second active materials comprise less than about 3% of themolar flux of ions passing through the ionomer membrane; (b) where theround trip current efficiency is greater than about 70%, greater thanabout 80%, or greater than about 90%: (c) where the round trip currentefficiency is greater than about 90%; (d) where the sign of the netionic charge of the first, second, or both active materials is the samein both oxidized and reduced forms of the active materials and matchesthat of the ionomer membrane; (e) where the ionomer membrane has athickness of less than about 100 μm, less than about 75 μm, less thanabout 50 μm, or less than about 250 μm; (f) where the flow battery iscapable of operating at a current density of greater than about 100mA/cm² with a round trip voltage efficiency of greater than about 60%;and (g) where the energy density of the electrolyte solutions is greaterthan about 10 Wh/L, greater than about 20 Wh/L, or greater than about 30Wh/L.

In some cases, a user may desire to provide higher charge or dischargevoltages than available from a single battery cell. In such cases,several battery cells can be connected in series such that the voltageof each cell is additive. This forms a bipolar stack. An electricallyconductive, but non-porous material (e.g., a bipolar plate) can beemployed to connect adjacent battery cells in a bipolar stack, whichallows for electron transport but prevents fluid or gas transportbetween adjacent cells. The positive electrode compartments and negativeelectrode compartments of individual cells can be fluidically connectedvia common positive and negative fluid manifolds in the stack. In thisway, individual cells can be stacked in series to yield a voltageappropriate for DC applications or conversion to AC applications.

In additional embodiments, the cells, cell stacks, or batteries can beincorporated into larger energy storage systems, suitably includingpiping and controls useful for operation of these large units. Piping,control, and other equipment suitable for such systems are known in theart, and can include, for example, piping and pumps in fluidcommunication with the respective chambers for moving electrolytesolutions into and out of the respective chambers and storage tanks forholding charged and discharged electrolytes. The cells, cell stacks, andbatteries of this disclosure can also include an operation managementsystem The operation management system can be any suitable controllerdevice, such as a computer or microprocessor, and can contain logiccircuitry that sets operation of any of the various valves, pumps,circulation loops, and the like.

In more specific embodiments, a flow battery system can include a flowbattery (including a cell or cell stack); storage tanks and piping forcontaining and transporting the electrolyte solutions; control hardwareand software (which may include safety systems); and a powerconditioning unit. The flow battery cell stack accomplishes theconversion of charging and discharging cycles and determines the peakpower. The storage tanks contain the positive and negative activematerials, such as the coordination complexes disclosed herein, and thetank volume determines the quantity of energy stored in the system Thecontrol software, hardware, and optional safety systems suitably includesensors, mitigation equipment and other electronic/hardware controls andsafeguards to ensure safe, autonomous, and efficient operation of theflow battery system A power conditioning unit can be used at the frontend of the energy storage system to convert incoming and outgoing powerto a voltage and current that is optimal for the energy storage systemor the application. For the example of an energy storage systemconnected to an electrical grid, in a charging cycle the powerconditioning unit can convert incoming AC electricity into DCelectricity at an appropriate voltage and current for the cell stack. Ina discharging cycle, the stack produces DC electrical power and thepower conditioning unit converts it to AC electrical power at theappropriate voltage and frequency fix grid applications.

Where not otherwise defined hereinabove or understood by one havingordinary skill in the art, the definitions in the following paragraphswill be applicable to the present disclosure.

As used herein, the term “energy density” will refer to the amount ofenergy that can be stored, per unit volume, in the active materials.Energy density refers to the theoretical energy density of energystorage and can be calculated by Equation 1:

Energy density=(26.8 A-h/mol)×OCV×[e ⁻]  (1)

where OCV is the open circuit potential at 50% state of charge, (26.8A-h/mol) is Faraday's constant, and [e⁻] is the concentration ofelectrons stored in the active material at 99% state of charge. In thecase that the active materials largely are an atomic or molecularspecies for both the positive and negative electrolyte, [e⁻] can becalculated by Equation 2 as:

[e ⁻]=[active materials]×N/2  (2)

where [active materials] is the molar concentration of the activematerial in either the negative or positive electrolyte, whichever islower, and N is the number of electrons transferred per molecule ofactive material. The related term “charge density” will refer to thetotal amount of charge that each electrolyte contains. For a givenelectrolyte, the charge density can be calculated by Equation 3

Charge density=(26.8 A-h/mol)×[active material]×N  (3)

where [active material] and N are as defined above.

As used herein the term “current density” will refer to the totalcurrent passed in an electrochemical cell divided by the geometric areaof the electrodes of the cell and is commonly reported in units ofmA/cm².

As used herein, the term “current efficiency” (I_(eff)) can be describedas the ratio of the total charge produced upon discharge of a cell tothe total charge passed during charging. The current efficiency can be afunction of the state of charge of the flow battery. In somenon-limiting embodiments, the current efficiency can be evaluated over astate of charge range of about 35% to about 60%.

As used herein, the term “voltage efficiency” can be described as theratio of the observed electrode potential, at a given current density,to the half-cell potential for that electrode (×100%). Voltageefficiencies can be described for a battery charging step, a dischargingstep, or a “round trip voltage efficiency.” The round trip voltageefficiency (V_(eff,rt)) at a given current density can be calculatedfrom the cell voltage at discharge (V_(discharge)) and the voltage atcharge (V_(charge)) using equation 4:

V _(eff,rt) =V _(discharge) /V _(charge)×100%  (4)

As used herein, the terms “negative electrode” and “positive electrode”are electrodes defined with respect to one another, such that thenegative electrode operates or is designed or intended to operate at apotential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to a reversible hydrogen electrode. The negativeelectrode is associated with a first electrolyte solution and thepositive electrode is associated with a second electrolyte solution, asdescribed herein. The electrolyte solutions associated with the negativeand positive electrodes may be described as negolytes and posolytes,respectively.

As indicated above, the present disclosure also provides methods forsynthesizing monosulfonated catecholate ligands and coordinationcomplexes containing such monosulfonated catecholate ligands.Optionally, the present disclosure further provides for isolation and/orpurification of the monosulfonated catecholate ligands and/orcoordination complexes formed therefrom. Further disclosure in thisregard is provided hereinafter.

In various embodiments, methods for synthesizing monosulfonatedcatecholate ligands can include providing a neat mixture of catechol andup to about 1.3 stoichiometric equivalents of sulfuric acid, and heatingthe neat mixture at a temperature of about 80° C. or above to form areaction product containing 3,4-dihydroxybenzenesulfonic acid(4-catecholsulfonic acid) or a salt thereof. In some embodiments, lessthan about 5% of the catechol is converted into 2,3-dihydroxysulfonicacid or a salt thereof.

In more specific embodiments, the neat mixture of catechol and sulfuricacid can be heated to a temperature up to about 120° C. or a temperatureof up to about 130° C. In general, lower reaction temperatures canincrease the proportion of 2,3-dihydroxybenzenesulfonic acid(3-catecholsulfonic acid) that is formed as a side product. Above about80° C., formation of this side product is not significant. If thereaction temperature is too high, some catechol can sublime from theneat mixture and increase the stoichiometric ratio of sulfuric acid tocatechol accordingly, thereby increasing the tendency of the reaction toform a disulfonated catecholate ligand (e.g.,4,5-dihydroxy-1,3-benzenedisulfonic acid). Accordingly, in moreparticular embodiments, the neat mixture can be heated at a temperatureranging between about 80° C. and about 110° C., or at a temperatureranging between about 80° C. and about 100° C. Closed reaction vessels,such as sealed tubes and pressure bombs, can allow even highertemperatures to be utilized.

In other more specific embodiments, the neat mixture can contain betweenabout 0.8 and about 1.2 stoichiometric equivalents of sulfuric acid.Even when a slight stoichiometric excess of sulfuric acid is present,excessive formation of a disulfonated catecholate ligand is notgenerally an issue in the synthetic methods disclosed herein.

In still other more specific embodiments, the neat mixture can contain asub-stoichiometric amount of sulfuric acid relative to the catechol. Insuch embodiments, the synthetic methods can be used to intentionallyproduce a reaction product containing unreacted catechol and3,4-dihydroxybenzenesulfonic acid or a salt thereof. In more particularembodiments, the neat mixture can contain about 0.5 stoichiometricequivalents of sulfuric acid or less relative to the catechol. In stillmore particular embodiments, the neat mixture can contain between about0.25 stoichiometric equivalents to about 0.5 stoichiometric equivalentsof sulfuric acid relative to catechol, or between about 0.3stoichiometric equivalents to about 0.5 stoichiometric equivalents, orbetween about 0.2 stoichiometric equivalents to about 0.4 stoichiometricequivalents. When a significant sub-stoichiometric amount of sulfuricacid is utilized in the synthetic processes of the present disclosure, amixture of unreacted catechol and the monosulfonated catecholate ligand(i.e., 3,4-dihydrobenzenesulfonic acid) can be produced. In someembodiments, the mixture of unreacted catechol and the monosulfonatedcatecholate ligand can be used directly without separating the catecholand the monosulfonated catecholate ligand from each other. For example,in the case of a 2:1 mixture of unreacted catechol and monosulfonatedcatecholate ligand being formed (i.e., 33% conversion of catechol to themonosulfonated catecholate ligand), a coordination complex bearing twounsubstituted catecholate ligands and one monosulfonated catecholateligand can be produced. Optionally, additional catechol ormonosulfonated catecholate ligand can be added to adjust thestoichiometric ratio for preparation of coordination complexes havingdifferent stoichiometries.

In further embodiments, methods of the present disclosure can includetreating the reaction product with a base before further utilizing the3,4-dihydroxbenzenesulfonic acid in the reaction product. In moreparticular embodiments, treating the reaction product with base caninclude neutralizing any excess sulfuric acid that may be present andreacting the sulfonic acid group to form a desired salt form. In variousembodiments, suitable bases for treating the reaction product andforming a desired salt form can include, for example, alkali metalhydroxides, alkali metal carbonates, alkali metal bicarbonates, ammoniumhydroxide, ammonium carbonate, and ammonium bicarbonate. In moreparticular embodiments, the base can be lithium hydroxide, sodiumhydroxide, potassium hydroxide, or any mixture thereof.

In more specific embodiments, methods of the present disclosure caninclude isolating the 3,4-dihydroxybenzenesulfonic acid or a saltthereof from the reaction product. More specifically, methods of thepresent disclosure can include neutralizing the reaction product with abase, and then isolating a salt of the 3,4-dihydroxybenzenesulfonicacid. In some embodiments, the 3,4-dihydroxybenzenesulfonic acid or asalt thereof can be isolated together in combination with unreactedcatechol.

In further embodiments, methods of the present disclosure can includereacting the 3,4-dihydroxybenzenesulfonic acid or a salt thereof with atransition metal compound to form a coordination complex having at leastone sulfonated catecholate ligand bound to a metal center. In moreparticular embodiments, the 3,4-dihydroxybenzenesulfonic acid can bereacted with a titanium compound, particularly a Ti(IV) compound, toform a titanium coordination complex containing at least one sulfonatedcatecholate ligand. In some embodiments, suitable titanium compounds caninclude titanium tetrachloride or titanium tetrakis(isopropoxide), forexample, which can be reacted under non-aqueous reaction conditions toform the titanium complex. In other embodiments, an acidic aqueoussolution of titanium oxychloride can be reacted with the3,4-dihydrobenzenesulfonic acid to form the titanium coordinationcomplex. The titanium oxychloride can be obtained commercially or can begenerated in situ by slowly adding titanium tetrachloride to water undercooling conditions (<0° C.) that do not result in substantial formationof titanium dioxide, the typical reaction product formed uponinteracting titanium tetrachloride with water. In still otherembodiments, titanium nanoparticles can be reacted with a sulfonatedcatechol compound to form a coordination complex.

EXAMPLES Example 1

Neat mixtures containing various stoichiometric ratios of catechol andsulfuric acid were prepared and reacted at various temperatures and forvarious lengths of time. Particular reaction conditions are summarizedin Table 1 below.

TABLE 1 ¹H NMR Ratio of Amount of Monosulfonated Disulfonated ReactionCatechol^(1,2) to Catechol Equiv. Temperture Time Other Unreacted(Estimated Entry H₂SO₄ (° C.) (hr) Conditions Catechol from ¹H NMR) 10.9 100 4 — 78:22 trace 2 1.05 100 17 — 91:8  7 3 1.05 85 17 — 76:24trace 4 0.9 100 17 3 Å 74:26 trace molecular sieves 5 0.9 100 5 4 Å61:38 trace molecular sieves 6 1.05 100 4 — 93:7  4 7 1.05 100 2 — 92:8 3 8 1.2 100 4 — 97:3  6 9 1.05 100 3 flowing N₂ 59:41 4 10 1.05 100 3 —92:8  3 11 1.05 100 0.5 — 84:16 6 12 0.33 85 3 — 32:68 trace to none 130.33 95 1 — 30:70 trace to none 14 0.33 115 2 — 32:68 trace to none 150.33 125 2 — 33:67 trace to none ¹3,4-dihydrobenzenesulfonic acid ²<5%2,3-dihydroxybenzenesulfonic acid was detected by ¹H NMRUpon discontinuing heating, the reaction mixture was added to anice/water mixture and was extracted 3 times with toluene. The aqueousphase was then evaporated to dryness, and 50% aqueous NaOH was thenadded to the resulting solid. The basic solution was then evaporated todryness a second time. The solid was triturated successively with hottoluene and with methanol, each of which was then removed bydecantation. The solids were filtered, washed with methanol and dried.In some instances, a second crop of product was recovered from thefiltrate. In still further instances, the product was recrystallizedfrom ethanol. FIG. 2 shows an illustrative ¹H NMR spectrum in D₂O of thearomatic region of 3,4-dihydroxybenzenesulfonic acid followingpurification.

As shown in Table 1, high ratios of the monosulfonated catecholateligand relative to unreacted catechol resulted when a slight deficit toa slight stoichiometric excess of sulfuric acid was reacted withcatechol. In contrast, when catechol was significantly present in excess(Entries 12-15), near-complete stoichiometric conversion of the sulfuricacid to the monosulfonated catecholate product occurred along with acorresponding amount of unreacted catechol. As discussed herein, themixture of the monosulfonated catecholate ligand and catechol can befurther processed to isolate the monosulfonated catecholate ligand orreacted directly with a transition metal compound to form a coordinationcomplex containing a mixture of catecholate and monosulfonatedcatecholate ligands (see Example 2).

Example 2

A mixture containing 2 equivalents of catechol and 1 equivalent of3,4-dihydroxybenzenesulfonic acid was mixed with methanol, and titaniumtetrakis(isopropoxide) was added slowly over a period of time.Distillation was conducted upon completion of the addition, and aqueousbase was added to form a corresponding salt of the sulfonatedcatecholate complex in an aqueous solution. For example, addition of anequimolar mixture of aqueous sodium hydroxide and potassium hydroxideproduced a mixed sodium/potassium salt of the sulfonated catecholatecomplex. FIG. 3 shows an illustrative ¹H NMR spectrum in D₂O of thearomatic region of the titanium complex formed from 2 equivalents ofcatechol and 1 equivalent of 3,4-dihydroxybenzenesulfonic acid.

Although the disclosure has been described with reference to thedisclosed embodiments, those skilled in the art will readily appreciatethat these are only illustrative of the disclosure. It should beunderstood that various modifications can be made without departing fromthe spirit of the disclosure. The disclosure can be modified toincorporate any number of variations, alterations, substitutions orequivalent arrangements not heretofore described, but which arecommensurate with the spirit and scope of the disclosure. Additionally,while various embodiments of the disclosure have been described, it isto be understood that aspects of the disclosure may include only some ofthe described embodiments. Accordingly, the disclosure is not to be seenas limited by the foregoing description.

What is claimed is:
 1. A composition comprising: a coordination complexhaving at least one monosulfonated catecholate ligand or a salt thereofbound to a titanium metal center; wherein the at least onemonosulfonated catecholate ligand has a structure of


2. The composition of claim 1, wherein the coordination complex has aformula of:D_(g)Ti(L₁)(L₂)(L₃) wherein: D is a counterion that is H⁺, NH₄ ⁺, Li⁺,Na⁺, K⁺, or any combination thereof; g ranges between 3 and 6; and L₁,L₂, and L₃ are ligands, wherein at least one of L₁, L₂, and L₃ is amonosulfonated catecholate ligand.
 3. The composition of claim 2,wherein each of L₁, L₂, and L₃ are monosulfonated catecholate ligandsand g is 5 or
 6. 4. The composition of claim 2, wherein one of L₁, L₂,and L₃ is a monosulfonated catecholate ligand and g is 3 or
 4. 5. Thecomposition of claim 4, wherein two of L₁, L₂, and L₃ are non-sulfonatedcatecholate ligands.
 6. The composition of claim 5, wherein two of L₁,L₂, and L₃ are unsubstituted catecholate ligands.
 7. The composition ofclaim 2, wherein the coordination complex comprises both Na⁺ and K⁺counterions.
 8. The composition of claim 7, wherein the coordinationcomplex has a formula of:Na_(x)K_(y)Ti(L₁)(L₂)(L₃) wherein: 3≤x+y≤6; at least one of L₁, L₂ andL₃ is a monosulfonated catecholate ligand; and x and y are each greaterthan 0, and are the same or different.
 9. The composition of claim 8,wherein comprising substantially equimolar amounts of Na⁺ and K⁺. 10.The composition of claim 2, wherein two of L₁, L₂, and L₃ aremonosulfonated catecholate ligands, one of L₁, L₂, and L₃ is anon-sulfonated catecholate ligand, and g is 4 or
 5. 11. The compositionof claim 10, wherein one of L₁, L₂, and L₃ is an unsubstitutedcatecholate ligand.
 12. The composition of claim 1 that is an aqueoussolution in which the coordination complex is dissolved.
 13. Thecomposition of claim 12, wherein the aqueous solution has an alkalinepH.
 14. The composition of claim 1, wherein the coordination complex hasan overall negative charge and comprises both Na⁺ and K⁺ counterions.15. The composition of claim 2 that is an aqueous solution in which thecoordination complex is dissolved.
 16. The composition of claim 15,wherein the aqueous solution has an alkaline pH.
 17. A flow batterycomprising a first half-cell and a second half-cell, wherein the firsthalf-cell contains a first electrolyte solution therein, the firstelectrolyte solution comprising the composition of claim
 12. 18. Theflow battery of claim 17, wherein the first electrolyte solutioncomprises the aqueous solution of claim
 15. 19. The flow battery ofclaim 17, further comprising a second half-cell having a secondelectrolyte solution therein, the second electrolyte solution comprisingan aqueous solution comprising an iron hexacyanide complex.